Cold forming by rolling of parts made of press sintered material

ABSTRACT

Method of cold forming by rolling of a blank made of press sintered material, in which at least one tool of predetermined peripheral geometry is brought close to the blank, so that the tool can then be rolled over the blank, urging the one towards the other, characterized in that it comprises, after a phase (a) of approaching the blank, a penetration phase (b), with (b n ) at least one phase of rolling under roughly constant load, as far as a chosen position, this load, the chosen position, and the corresponding number of passes being determined so as to control the surface densification and the dimensions of the rolled part.

[0001] The invention relates to the cold forming of parts from blanks,particularly metal blanks. It applies in particular to blanks made ofpress sintered material.

[0002] “Cold forming” is to be understood as meaning deformation of themetal of the blank at ambient temperature or in the semi-hot state (upto a temperature of 300 to 500° C. depending on the metal of the blank),below its melting point.

[0003] A distinction should be drawn between cold forming by revolutionrolling (“rolling” for short), which uses rotary tools or theequivalent, as opposed to other methods of cold forming such asmachining, drop forging, stamping or extension.

[0004] There are several configurations for forming by rolling:

[0005] external forming of the blank, using a tool, the blank being heldin some other way, or alternatively using two or more tools distributeduniformly around the external periphery of the blank;

[0006] internal forming of a hollow blank, using at least one internaltool and at least one external tool, or an external support which turnswith the blank.

[0007] Furthermore, the blank is often driven by the tool or tools, butcan also be driven separately, in synchronism or otherwise.

[0008] Mastering the position of the tools with respect to the blank isa particularly tricky operation. Use is generally made of hydraulic(ram) or mechanical (screw-nut) position control. However, it has becomeapparent that the known control techniques were not always satisfactory,particularly in the case of blanks made of press sintered material, aswill be seen.

[0009] The present invention improves the situation.

[0010] As proposed, the method of cold forming by rolling of a blankmade of press sintered material is of the type in which at least onetool of predetermined peripheral geometry is brought close to the blank,so that the tool can then be rolled over the blank, urging the onetowards the other.

[0011] According to one aspect of the invention, this method comprises,after a phase (a) of approaching the blank, a penetration phase (b),with:

[0012] (b_(n)) at least one phase of rolling under roughly constantload, as far as a chosen position, this load, the chosen position, andthe corresponding number of passes being determined so as to control thesurface densification and the dimensions of the rolled part.

[0013] According to another aspect of the invention, after a phase (a)of approaching the blank, a penetration phase (b) is provided, with:

[0014] (b_(n)) at least one phase in which the rolling load increases,bounded by a maximum value of this rolling load.

[0015] The phase (b_(n)) of rolling under roughly constant load can thentake place, as appropriate.

[0016] Other features and advantages of the invention will becomeapparent from examining the detailed description which follows, and theappended drawings in which:

[0017]FIG. 1 schematically depicts a cold forming machine, having afirst type of tool drive,

[0018]FIG. 2 schematically depicts an alternative form that applies inparticular to the machine of FIG. 1,

[0019]FIG. 3 schematically depicts a cold forming machine with a secondtype of tool drive,

[0020]FIG. 4 schematically and partially depicts a machine of the sametype as that of FIG. 1, but in which one of the tools works inside anannular blank,

[0021]FIGS. 5A to 5G illustrate various alternative forms of thegeometric arrangement of the forming tools,

[0022]FIG. 6 is a flow diagram of a known machine control, usingposition control,

[0023]FIG. 7 is a flow diagram of a machine control used according tothe invention, with force control,

[0024]FIG. 8 is a diagram of steps illustrating an exemplaryimplementation of the invention,

[0025]FIGS. 9A and 9B are schematic time charts of force and positionrespectively, in one example of an application of the invention,

[0026] FIGS. 10 to 13 are measured force and position diagrams invarious exemplary implementations of the invention, and

[0027]FIG. 14 schematically illustrates a blank and a part for oneparticular example of rolling.

[0028] Furthermore, Annex 1 expresses, in the form of a table,characteristics of the control of cold forming machines according to theinvention.

[0029] The detailed description hereafter, the annex or annexes, and thedrawings contain, essentially, elements of a certain nature. They can beused therefore not only to allow a better understanding of thedescription, but also to contribute to defining the invention, asappropriate.

[0030] Cold forming makes it possible in particular to produce a veryprecise shape (forming proper) and/or to alter a surface finish, whichis often known as roller burnishing or alternatively “superfinishing”.

[0031] Conventionally, the blank is the thing which enters the formingmachine, with or without preform, and the part is the thing which leavesit. The words “blank” and “part” will be used arbitrarily or togetherfor the intermediate states inside the machine.

[0032] Detailed information on cold forming by revolution rolling, orrolling, can be found on the site www.escofier.com, on the pages“metier_procédé”, and in the corresponding printed technicaldocumentation.

[0033] The invention relates a priori to methods employing machines withvariable distances between centres, with tools of roughly constantprofile on their periphery, and working with “plunge feed”, that is tosay which move closer to the part or blank. This differs from machinesof the “Incremental” (registered trade name) type, which have tools witha variable, generally progressive, profile on their periphery, andoperate with a fixed distance between centres, that is to say withoutany relative movement of the axes of revolution of the tools and of thepart moving closer together, or machines which operate successively,involving circulating the part axially with respect to the tools, theworking distance between centres of which is constant.

[0034]FIG. 1 relates to a rolling machine with two tools O1 and O2,which operate on a blank for forming EB (which may also be termed“part”). The machine comprises, on a general frame (not depicted), twohalf-frames F1 and F2, which support, in rotation, the tools O1 and O2about roughly parallel axes A1, A2. A motor M1, for example an electricmotor, drives two worm/threaded roller systems SCR1-G1 and SCR2-G2 (orjust one system), the output movement of which is applied to the toolsO1 and O2 to make them rotate in the same direction in sychronism. Theaxes A1 and A2 define the respective reference axes of the tools forforming the blank. The machine comprises, on the general frame, asupport (not depicted) for the blank EB, so that it can move in terms ofrotation, in the opposite direction to the tools, about an axis roughlycoplanar with the two axes of rotation A1 and A2.

[0035] The two half-frames F1 and F2 can move with respect to oneanother, in this instance under the effect of a ram system having apiston P1 and cylinder C1, which is placed on one of the half-frames,while the end of the piston rod is fixed at P10 to the other half-frame.The lateral nature of this control may be compensated for by amechanical equalizer, not depicted.

[0036] The machine further comprises, illustrated schematically, asensor XS that senses the relative position of the two half-frames,therefore of the axes A1 and A2. The two chambers of the ram, one oneach side of the piston P1, are fed with fluid from a hydraulic unit HG,through a servovalve SV. The latter is operated by a numericallycontrolled controller NC. The controller NC receives an indication ofthe pressures Pa, Pb in the two chambers of the ram. It also receives anindication of the position X from the sensor XS. It sends the servovalvea command SVC, in correspondence with program data PRG and its inputs.

[0037]FIG. 1 corresponds for example to machines of the series Hxx CN ofESCOFIER TECHNOLOGIE, where xx corresponds to two figures indicating asize.

[0038] Having fitted the blank, the two axes A1 and A2 far enough apart,the program data is implemented to carry out the forming of the blankEB, by relative advancement of the axes A1 and A2, bearing in mind theperipheral geometry of the tools, and many other parameters. Three mainphases can be distinguished from within the forming process. These are:penetration, sizing, decompression.

[0039] The machine depicted schematically in FIG. 2 is of the same kind,except that instead of being moved only by the tools O1 and O2, theblank is positively driven by a motor M2, for example an electric motor.This alternative form can also apply to the embodiments which follow.

[0040] This additional or supplementary drive of the part with respectto the tools can also be implemented when the circumstances or themethod so dictate (automatic indexing of toothed parts, precise divisionof profiles, in particular):

[0041] independent drive in the machine known as “H40 CN galetage” [anumerically controlled roller burnishing machine] from ESCOFIERTECHNOLOGIE,

[0042] synchronized drive in the machine known as “Syncroll” fromESCOFIER TECHNOLOGIE.

[0043] The motor M2 is then kept synchronized as wished with the motorM1, particularly bearing in mind the required synchronization ratio.This ratio may be taken between the angular velocity ω1 of the tools andthat ω2 of the blank, more exactly to preserve the equality of theirrespective tangential speeds at their operating diameters. In the caseof a toothed profile, a number-of-teeth ratio may be taken.

[0044]FIG. 3 is similar to FIG. 1 and the driving of the tools O1 and O2is not repeated. The difference lies in the fact that the general frameB is shown, and the half-frames F1 and F2 are mounted on it viascrew/nut drive systems BSD1 and BSD2 which are actuated, by twohomologous transmissions, from an electric motor M3 fixed to the frame.The controller NC receives values regarding the state of the motor, inparticular information regarding the angular velocity (ω) and theposition of the rotor (α); it controls the motor M3 accordingly, on thebasis of program data PRG, and of the instantaneous position X, which isa function of the angular position α.

[0045]FIG. 3 corresponds, for example, to machines of the NT series fromESCOFIER TECHNOLOGIE.

[0046]FIG. 4 partially illustrates another alternative form. Here, theblank EB, which is annular, is housed in a part holder PP and the toolO1 is on the inside, driven by the motor M1, while on the outside, aroller G, driven in rotation by contact with the part holder, allows therolling load to be applied. In the example, there is now just onecarriage F2 housed in the general frame B. The position sensor XS is onthe inside, between F2 and B. The control elements in FIG. 1 (SV, HG,NC, PRG) can be read across to FIG. 4, only the servovalve SV (orequivalent) being depicted in FIG. 4.

[0047] An alternative form of FIG. 4 consists in using, for drive, oneof the drive systems of FIG. 3, for example the one illustrated as BSD1,and its auxiliaries, with the corresponding control elements (M3, NC,PRG).

[0048]FIG. 4 corresponds for example to the machines of the ALS seriesfrom ESCOFIER TECHNOLOGIE.

[0049] In general, it is the tools which drive the rotation of the part.However, the part may also be driven, as in the case of FIG. 2. Amachine may have from one to n tools, of which the configuration, thatis to say the geometric layout and support, can know variousalternatives:

[0050] two external tools, both able to move in relative translation(FIG. 5A) as already described with regard to FIGS. 1 to 3;

[0051] two external tools of which one, O1, is of fixed axis, and theother, O2, of an axis that can move in translation (FIG. 5B);

[0052] more than two external tools, in theory uniformly distributed,able to move in relative translation, for example 3 tools (FIG. 5C) or 4tools (FIG. 5D);

[0053] one tool on the inside and the other on the outside, for anannular blank (FIG. 5E);

[0054] alternative forms with just one tool, which may be on the inside(FIG. 5F), the blank being held on a support EBS which is able to movein terms of rotation, as described, for example, with regard to FIG. 4,or on the outside (FIG. 5G), the blank being mounted on a rotarysupport.

[0055] Furthermore, various tool peripheral geometries are used,particularly for forming splines, knurling, screw threads, gearing, orany other shape on a cylindrical base.

[0056] In the text which follows, the term “the tools” will be usedarbitrarily to denote one or more tools, which are also known as“knurling wheels”.

[0057] In these machines, the action of deforming the part using thetools is the consequence of the relative radial movement created betweenthem by a movement device. As has been seen, this may be a hydraulicmeans (ram) or a mechanical means (screw/nut system associated with anelectric or hydraulic motor). It is also possible to imagine linearmotors.

[0058] The deformation of the part, in accordance with the peripheralshape of the tools, demands action which varies according to numerousparameters or factors:

[0059] the materials of the part and of the tools,

[0060] the shapes produced,

[0061] the respective diameters of the part and of the tools (or someother critical dimension),

[0062] the area of contact between the part and each tool, resultingfrom the depth of penetration upon each action or pass of the tools.

[0063] The physical quantities relating to the above parameters(material hardness, contact area, rate of penetration of the tools intothe part, etc.) determine at each moment the necessary and sufficientresultant load involved in the deformation.

[0064] This load needs to become high enough and to be applied for longenough (number of revolutions of the parts) to achieve the desireddeformation, without causing the part to break or creating defects thatmake it unfit for use. If the load needs to be changed, it will benecessary to change one of the physical quantities, which will often bethe rate of penetration of the tools into the part.

[0065] As the part is being rolled, its resistance to local deformationat the point at which it is in contact with the tools increases, forvarious reasons, including a phenomenon of the work hardening of thematerial, brought about by the successive deformations caused each timethe tools and the part come into contact.

[0066] The pressure needed for deformation therefore increases withthese phenomena.

[0067] The area deformed by the tool also increases as rollingprogresses, whereas the blank little by little adopts the shape that isthe conjugate of the tool or tools.

[0068] The rolling load is the product of the contact pressure of thetool or tools and the area on which these act. Assuming (for simplicity)a constant rate of tool penetration, the rolling load thereforeincreases as rolling progresses, and does so at least as quickly as, andgenerally more quickly than, this penetration rate.

[0069] In order for it to be possible to apply the required localdeformation, it is necessary that, from start to finish, the part in itsentirety should withstand the total force that the tools impart to it,until a final part is obtained that meets the desired geometric andstructural criteria for this stage of its manufacture.

[0070] At the same time, the rolling tools are subjected to high loads.The intensity and the repetitiveness of these loads determine the lengthof time for which a tool can be used. In turn, the cost of the tool isan important factor in the cost of the rolling operation, and may evencompromise its viability or competitiveness.

[0071] The aforementioned machines generally work on blanks of partsmade of solid metal. The control loops which control the pressing of thetools on the part are controlled in terms of position, and apply therequired load—whatever this may be—to maintain the anticipated relativeposition of tools and part at every moment during the forming process.There does not currently exist any model which makes it possible torepresent the phenomenon, even for a solid material. In consequence, thecontrol programs are written experimentally.

[0072] Various factors mean that it is sometimes desirable to use blanksmade of press sintered material.

[0073] “Press sintered blank” is to be understood as meaning a partobtained in an earlier stage by sintering metal powders, that is to saya part whose relative density is still less than 100%. A press sinteredblank can be obtained by uni-axial mechanical pressing of powders, andsolid phase sintering. The blanks thus obtained are incompletelydensified, their density ranging from 80 to 95% of that of a solidmaterial (relative density), typically from 90 to 92%.

[0074] Parts obtained directly by press sintering are very economical toproduce. However, the dimensional accuracy on their shape may beinsufficient for certain demanding applications. In addition, problemsmay arise regarding the in-service integrity of highly stressed regions,because of the incomplete densification of the parts made of presssintered material.

[0075] At the present time, little or no use is made of cold forming ofunformed press sintered blanks, in spite of the various proposals thatexist:

[0076] U.S. Pat. No. 5,711,187 and U.S. Pat. No. 5,884,527 describesurface re-machining of sintered gears already preformed, byconventional rolling, that is to say without worrying about or applyingspecial teaching to the rolling conditions and their consequences;

[0077] U.S. Pat. No. 5,659,955 also starts out with sintered blanks, andperforms on them either machining which progresses lengthwise (in thedirection of the axis of rotation of the blank) or, here again, surfacere-machining of already preformed sintered gears, the principle of whichis of the “sequential” type, on a machine with fixed distances betweencentres;

[0078] other patents, such as U.S. Pat. No. 4,708,912 or alternativelyDE-A-3 140 189 attempt to apply conventional rolling, essentially toobtain highly stressed gears.

[0079] The Applicant Company has once again invested interest in therolling of parts made of press sintered material. It has observed that,when a rolling technique is applied to press sintered blanks, the limitsand conditions on the production of the parts from these blanks differgreatly from those which would be encountered in respect of identicalparts rolled from a solid blank made of the same material. What happensis that the density and the strength of the press sintered material arebelow those of the solid material, and the spread on the dimensionalcharacteristics of the blanks is wider, particularly in terms ofeccentricity and circular symmetry.

[0080] It has been observed in particular that:

[0081] the core of the press sintered part has lower resistance to thevarious mechanical stress than a solid part,

[0082] whereas the surface pressure needed for deformation will increasewith the peripheral densification resulting from this deformation, untilit reaches a value close to that of the solid material.

[0083] Here, “densification thickness” is used to denote the distancebetween the outer surface of the part and its boundary on the core side,where the press sintered material maintains the initial density of theblank (the density obtained in the last sintering operation).

[0084] With conventional rolling techniques, it has been found that this“densification thickness” is small, generally less than 1 mm. This maybe enough to improve the in-surface integrity of fairly highly stressedgears. This is what is described in patents U.S. Pat. No. 5,711,187 orU.S. Pat. No. 5,884,527, which show that densification of 90 to 100%over a thickness of 0.38 to 1 mm at the root of the teeth and/or on theflanks of the teeth of a gear may prove suitable, without, however,describing precisely how to achieve this industrially.

[0085] In general, this densified layer, which is locally stronger, isnot enough to withstand the overall deformation load when this becomeshigh; next, the core is not itself strong enough. This gives rise tovarious forms of deterioration. In particular, it results in local orcomplete rupturing of the part, for example starting from the core, inthe case of solid parts, or starting from the surfaces, in the case ofball bearing rings. The Applicant Company has observed that excessivetri-axial stresses arise on regions which are insufficiently able towithstand rupture because they are not completely densified. There havealso been observed phenomena of the material collapsing or of itbreaking up at the surface, which then falls off as dust or smallfragments and makes continuing to roll impossible. Also observed hasbeen instability of the deformation, which seems to be specific to presssintered material; the result of this is that the setting of theconventional rolling parameters determined on a given blank may not suitthe next blanks, leading to random and therefore unacceptable results,because of their lack of reproducibility in the face of the tolerancesinherent to the production of blanks using press sintering.

[0086] In other words, because of its granular nature, the presssintered material has significant variations in homogeneity, which areexacerbated by the method of manufacture of the blanks. These variationsare wide enough to contribute to increasing the difficulties inmastering the rolling conditions, as needed for generating parts whichmeet the geometric and functional desires of the user.

[0087] Furthermore, a rolling technique applied to sintered materialswill have, on the tools, the same effect as it has when applied to asolid material subjecting it to the same rolling stresses, particularlyon the length of time for which they can be used. It is clear that theaforementioned difficulties, particularly the risks of parts rupturing,are of a kind that will markedly reduce tool life.

[0088] The Applicant Company has observed that it is possible to improvethings by taking an approach which is the opposite of the approachhitherto taken.

[0089] Conventionally, in the case of a servovalve or servodistributor(FIG. 1), NC-type control is performed as indicated schematically inFIG. 6. The output stage NC90 which controls the servovalve is itselfcontrolled by a stage NC10 which defines the throughput of theservovalve as a function of the current position X, and possibly as afunction of its previous values (or its derivative). The action istherefore in fact on the rate of advance of the tool or tools andtherefore on the positions.

[0090] It is possible to proceed in a different way, as indicatedschematically in FIG. 7. The output stage NC90 which controls theservovalve or the servodistributor is itself controlled by a stage NC20which defines a variation in the throughput of the servovalve or of theservodistributor, so as to control the forces or loads transmitted tothe part or blank during the rolling cycle, as a function of the currentload. In this particular instance, this load is calculated from valuesfrom pressure sensors, such as the aforementioned Pa, Pb, bearing inmind the areas exposed to the fluid on each side of the piston. The loadcan thus be measured.

[0091] It has been found that this improves the rolling characteristics,and the characteristics of the part obtained; furthermore, it extendsthe life of the tools, and preserves their integrity while at the sametime avoiding overload.

[0092] In other words, with a view to rolling sintered parts, there isproposed a method of rolling with controlled loads or forces, preferablyvia a control loop or, more generally, feedback.

[0093] As with positional control of the prior art, the behaviour of ablank subjected to rolling under force or load control cannot currentlybe modelled, even for a solid material, and especially for a presssintered material. In consequence, the control programs are writtenexperimentally, at least in respect of phase (b).

[0094] In FIG. 7, control of the load delivered by the rolling means(machine) is obtained with reference to automatic control of a toolmovement hydraulic system. The person skilled in the art knows how totranspose such automatic control to other movement systems, particularlyan electric motors system as illustrated in FIG. 3.

[0095] The means for measuring physical variables of load and position,needed for automatic control and control, such as, for example, distancetravelled, angle of rotation of a screw-nut system, pressure of a fluid,strength voltage frequency of a current, strain on a correspondinggauge, are chosen in accordance with the solutions adopted for thedesign of the various types of machine concerned.

[0096] It has been found that load or force (automatic) control ismarkedly superior to position (automatic) control. In the absence of amodel, the phenomena involved are difficult to analyse. It would,however, seem that this superiority stems in part from the fact thatload or force control gives better tolerance towards any problems theremight be which are associated with press sintered blanks, given theresponse of the control sequence. This superiority to a great extentcompensates for the disadvantages associated paradoxically with the useof load or force control even though the desired end result is that ofobtaining a precise position (in absolute or relative terms).

[0097] Furthermore, it has been found that, all other things beingequal, the densification thickness obtained with “load” control isgenerally a little greater than that obtained with position control.This seems to be due to better “regularity” of the rolling action, inthe presence of imperfections. At the same time, work hardening can becontrolled better. The same is true of the effects of the variations inambient temperature on the machine, and of the temperature of itsinternals, particularly the motor elements (such as the fluid). The sameis also true of the effects of the surface heating of the part or blank,which are also better controlled. Furthermore, this heating is not assignificant, because of the better control over the work hardening.

[0098] In greater detail, the following advantages have appeared forload (automatic) control. It makes it possible:

[0099] a) no longer to be subject to the variations in the actualposition of the tools, with respect to the measured position,consequences of the variations in load on mechanical elements which(machine included) in fact behave like big springs (of spring rate K),

[0100] b) to be able to enjoy the full benefit at the start of rollingof the advantages of a material which has not yet been affected by workhardening, which is useful for the final mechanical strength butunfavourable in terms of deformation and its progressiveness and tendsto give rise to local heating of the part or blank.

[0101] The difficulty stems not only from the effects of smallirregularities of all kinds, but also from the fact that the interactionbetween a given region of the blank and the active tools takes place ina “chopped” way, n times per blank revolution, where n is the number ofactive tools.

[0102] One exemplary embodiment will now be described with reference toFIG. 8, and to Table 1 of Annex 1. In this Table 1, the grey boxesindicate, in each phase, the essential quantity or quantities on whichthe numerical control relies. The tools are constantly rotating, asindicated in the angular velocity column ω.

[0103] In this example, rolling cycles comprising the operationsdescribed hereinafter are implemented.

[0104] In general, the positions X are considered to be decreasing whenthe tools are approaching the part (because the tools are thenapproaching one another, and at the same time approaching the part).

[0105] An initial approach operation 80 or (a₀), not represented inTable 1, may be carried out in any desired way, until the tools are in aposition a short distance away from the part.

[0106] Next, phase 82 or (a₁)—(a) in Table 1—achieves contact with thepart. It comprises a slow advance at a speed Ca, and under light loadFa, associated with the movement of the carriages. Contact is achievedby looking for the position Xa, lying between Xa₁ and Xa₂, for which theload needed to advance increases substantially to a value Fa₁,indicating contact between tools and part. The advance can be bounded bya minimal position XMINa.

[0107] The threshold load value Fa₁ is suitably adjusted to avoid thetools making a damaging imprint in the part upon first contact. Thisadjustment is trickier to achieve with a press sintered blank and mayneed to be performed by prior iterations, during optimization tests.

[0108] It is important to note that, upon contact, the part begins toturn (except perhaps when it is moved separately).

[0109] In operation (b) or 84+86, the load applied to the tools, as aresult of their relative movement with respect to the part, thenincreases progressively and in a controlled fashion to a desired levelFb. A limit is set on the advance X, to avoid the possible disastrousconsequences of the press sintered material beginning to collapse in onitself (like a human foot would cause on uncompacted snow). Here,“collapse” is to be understood as meaning an unexpected sharp change inposition.

[0110] As a preference, the initial phase or phases 84 of operation (b)are carried out with one or more levels of rate of load increase. In theexample depicted, two rates of increase are envisaged, these being equalapproximately to (Fb₁−Fa₁)/Tb₁, then (Fb₂−Fb₁)/Tb₂, to reach the levelsFb₁ and Fb₂, respectively.

[0111] Thus, the progression of the load applied to the blank can bekept below a defined limit value, so as not to cause a critical state toarise during deformation (in particular so as not to initiate theaforementioned collapse). That being so, the increase in load is chosento be as rapid as possible, so as to limit the effects of work hardeningas a result of the successive contacts between the tool and the part.What happens is that excessive work hardening results in superficialhardening, which causes the load required to continue forming toincrease and therefore also increases the risk of running into acritical state, it being remembered that the blanks have dimensionaltolerances, surface irregularities and also intrinsic inhomogeneity.

[0112] These initial phases (b) are important for obtaining deformationwhich is as uniform and progressive as possible at the start of rollingwhereas, in particular:

[0113] The sintered blank always has defects in dimension, roundness,concentricity and homogeneity

[0114] The depth of action of the tools in the part changes between azero value (contact between the tools and the part at the start) and adepth that results from their progressive penetration as the partrotates before the original point of contact once again meets the tools(half a revolution of a part, for example, on a machine with twoknurling wheels)

[0115] The mechanical parts of the machine experience variabledeformations in relation to the variation of the working load.

[0116] In the final phase 86 of operation (b), the load is thencontrolled (F) for one or more successive phases, so that its changecontinues to observe a predefined cycle, until a final relative position(Xb) of the tools and of the part is reached which tallies with thefinal dimension of the part.

[0117] The simplest solution is a single phase with automatic controlunder constant load (Xb=Xb₂) until the desired final position isobtained. Conversely, the most complicated solution may be a successionof phases with load control changing progressively, uniformly, or insuccessive levels, in a controlled way. In general, theautomatically-controlled load values Fb remain close to the load Fb₂achieved at the end of step b₂ (or more generally b_(n)).

[0118] It is also possible to include intermediate phases in order, forexample, to change the direction of rotation of the tools. Of course,numerous intermediate solutions are conceivable.

[0119] In all the load controlled phases of operations (b) and/or (c),the relative position of the tools and of the part at each moment “t” isa consequence of the control over the controlled load, as programmed upto that moment “t”.

[0120] During phase (b), generally known as “penetration”, surfacedensification of the blank is achieved over a chosen densificationthickness. This densification thickness depends on the density of theblank before rolling, on the nature of the material of which it is made,and on the geometric modification imposed by the tools during rolling,bearing in mind the controlled load values applied. Here again, theconditions required for obtaining a chosen densification thickness maybe determined by tests beforehand.

[0121] Optionally, a final sizing phase, denoted 88 or (c) may beperformed. This phase may use position control, to set a tool/partrelative position (Xc). This may, for example, make it possible toobtain a part which meets roundness criteria predefined by the user. Theload is no longer the basic quantity for the automatic control in thisphase, and varies generally in a roughly decreasing manner down to a lowvalue associated with the plastic deformation limit value, below whichthe part will experience only elastic deformation. In this finishingphase (c), the blank/tool relative position is kept roughly constant fora chosen length of time defined to obtain a part of acceptable geometry,particularly of accepted roundness.

[0122] In the last steps of the load control, it is necessary to masterthe transition with the following so as to avoid “position overshoot”and/or “load overshoot” which could compromise the quality of the part.

[0123] In principle, the periphery of the rolling tool or tools isroughly circular (in cross section) or generally cylindrical (withrespect to a mean diameter, in the presence of teeth, or of a screwthread). The blank may be preformed, particularly with teeth, in whichcase, in principle, the tool or tools are equipped with homologousteeth. As an alternative, the blank may be preformed as a ring,particularly the ring of a bearing, in which case, in principle, thetool or tools has a uniform external periphery (not necessarilycylindrical of revolution).

[0124] A terminal decompression phase (d) or 88 is provided, forwithdrawing the tools away from the part. This phase may be determinedconventionally in terms of retreat rate or better controlled, in theform of an automatically controlled decreasing load.

[0125] In the foregoing, the penetration phase or phases take placeunder load control. As the objective is to reach a programmed positionXb, the automatic control is ended when the desired position is reached(86). The whole thing can therefore be termed load/position (load thenposition) control.

[0126] Alternatives are conceivable. For example, load/excursion controlmay be performed, in which the load control is maintained until aprogrammed excursion or distance has been covered. In this case, thefinal position is a programmed consequence of the initial position(contact point), in relative terms, rather than as a position inabsolute terms. This may be used, for example, to reduce by a roughlyconstant value blanks which have a variable starting diameter. It isalso possible to envisage other conditions for load control such as, forexample, “load/time” control, with fixed time. This may in particular besuitable where the control of the diameter of the part is not critical,for example:

[0127] for special operations such as roller burnishing, oralternatively

[0128] when the cycle of rolling a blank contains several sub-cycles,with or without reversal of the direction of rotation betweensub-cycles, in the case of the sub-cycles preceding the final sub-cycle.

[0129] It has been seen that the method thus described performs coldforming by rolling of a blank made of press sintered material, in whichat least one tool of predetermined peripheral geometry is brought closeto the blank so that the tool can then be rolled over the blank, urgingthe one towards the other. After a phase (a) of approaching the blank,the method comprises a penetration phase (b).

[0130] According to one aspect of the invention, this penetration phasecomprises, towards its end (b_(n)), at least one phase of rolling underroughly constant load, as far as a chosen position, this load, thechosen position, and the corresponding number of passes being determinedso as to control the surface densification and the dimensions of therolled part. The roughly constant load may be defined with respect to acritical value, kept below the deterioration threshold, which can bedetermined experimentally and/or in some other way (for example byextrapolation from similar parts). The expression “roughly constant” isto be understood as meaning a variation which may be of the order of 10%of the critical value. The 10% are preferably taken under the criticalvalue, which may allow this critical value to be brought close to thedeterioration threshold, if so desired. In that very way, it is possibleto reduce the rolling time and then to have better control over the workhardening.

[0131] In other words, phase (b) may include keeping the load applied tothe blank below a limit value defined with respect to a threshold atwhich the press sintered blank deteriorates. The deterioration may stemfrom a rupturing of the core, breaking-up of the surface, and/or inducedwork hardening. The deterioration threshold depends on various factorssuch as the stresses that the blank can tolerate with respect to thedesired conformity of the finished part, and the stresses associatedwith the desired tool life. Phase (b) may also include keeping the loadapplied to the blank at a value close enough to the said limit to avoidexcessive work hardening while at the same time minimizing the rollingtime (on which the cost of production depends). However, there areapplications such as “roller burnishing” (which corrects the geometry ofa part), in which work hardening is not as critical, or may even bedesired.

[0132] According to another aspect of the invention, which can bedissociated from the previous one, the penetration phase (b) isperformed at least partially under load control.

[0133] According to yet another aspect of the invention, the phase(b_(n)) of rolling under roughly constant load may be preceded by (b₁)at least one phase in which the rolling load increases, bounded by amaximum value of this rolling load. It is currently preferable for theincrease in load in phase (b₁) also to be bounded in terms of theprogression of the load over time. More specifically still, the increasein load in phase (b₁) may be carried out according to a critical lawwhich tends to bring the progression close to an experimentallydetermined permissible limit value that takes account of the geometricand mechanical properties of the blank and of the finished part. Thismakes it possible to get close to the ideal situation which (except inspecial cases) consists in increasing the load as swiftly as thecharacteristics of the blank and of the finished part permit.

[0134] The periphery of the tools may be uniform or smooth, so as toform rings or bearing surfaces, something which is particularlyadvantageous with press sintered material since the material candensify, without spreading longitudinally in the direction of the axesA1 and A2, like a solid material would. Benefiting at least in part fromthe same advantage, it may also adopt other predetermined shapes: screwthreads, or annular grooves, or straight-cut or helical teeth,particularly to form splines, knurling, a screw thread or a gear.

[0135] Furthermore, the blanks may themselves comprise shapesoriginating from press sinter production, for example teeth.

[0136]FIGS. 9A and 9B illustrate general appearances of force andposition curves that can be seen according to the invention. Here, thereare two phases (b₁) and (b₂) which comprise, before the level F=Fb,different rates of increase of load, here constant and equalling:

[0137] (Fb₁−Fa₁)/Tb₁ and (Fb₂−Fb₁)/Tb₂.

[0138] FIGS. 10 to 13 illustrate actual position (scale on the left) andforce (scale on the right) curves. The increase in position on the rightcorresponds to the withdrawal of the tools, in phase (d). The followingcomments can be made on these diagrams:

[0139]FIG. 10: semi-rapid approach (a₀, a₁), rapid increase in load(b₁), rolling (b₂) under roughly constant load, no phase (c), very shortphase (d);

[0140]FIG. 11: differs from FIG. 10 by a more rapid approach (a₀, a₁),two-phase increase in load (b₁, b₂), starting slowly and then becomingmore rapid; rolling (b₃) under roughly constant load, no phase (c), veryshort phase (d);

[0141]FIG. 12: differs from FIG. 11 by an even more rapid approach (a₀,a₁); the increase in load (b₁, b₂) is also in two phases, with differentrates; phase (c) has an overall decreasing load, but with fluctuationsdue, in the presence of a fixed distance between centres, to the slightbut inevitable geometric imperfections upon tool/part contactparticularly regarding the roundness of the part (with two tools, agiven region of the part encounters a tool twice per revolution);

[0142]FIG. 13: generally similar to FIG. 10, but with a split into twoparts 1 a ₀ to 1 d and 2 a ₀ to 2 d; a reversal of the direction ofrotation of the tools may be performed between the two parts, at thestart of 2 a ₀. In other words, the approach phase (a) and thepenetration phase (b) are repeated after the direction of rotation ofthe tool or tools has been reversed. This may be performed severaltimes.

[0143]FIG. 14 is a schematic sectional view which shows the blank EB,and the finally desired part PI. The hatched region corresponds to thepart of the blank which is not modified by rolling and the cross-hatchedregion shows the final geometry of the part, whereas the blank hasslightly larger dimensions, as illustrated. Such a part is known by theterm “biconical roller” and may, for example, have a diameter of 30 mm(blank).

[0144] Such a part can be manufactured using a conventional method,using position control (expressed in speed and in final position), toobtain a final outside diameter of 29.5 mm. In practice, variations inexcess of 30μ on the final diameter and even a roundness defect inexcess of 30μ are observed. This is accompanied by surface workhardening.

[0145] This spread in the part obtained stems, on the one hand, from thespread in the diameters of the blank and, on the other hand, from thespread in the shape of the blanks (as regards the width of thecylindrical part and the width of the cones), and then again from aspread in hardness between one blank and another, and also finally fromvariations in homogeneity in the press sinter of which the blank ismade.

[0146] It has been found that the aforementioned spreads resulted invariations in the mean load applied when switching from one blank toanother, and in fluctuations in load in the course of a cycle, with agenerally continuous increase in the load applied.

[0147] There are secondary consequences which ensue from this, and theseare fluctuations in the actual distance between centres of the tools,with respect to the position-controlled distance between centres. Thiscan be expressed using a relationship of the form ΔX=f(F)/K, where K canbe considered as being the spring rate for the mechanical partsconcerned, in the rolling machine.

[0148] The same kind of part has been prepared by rolling according tothe invention, with load control, followed by final position control forthe super-finishing.

[0149] As far as the parts are concerned, the variation in end diameteris now at most equal to 15μ. The variations in roundness are at mostequal to 10μ. This clearly illustrates the advantages that can beobtained using the present invention, particularly better repeatability.

[0150] It has also been found that, for the same cycle time, the meanload obtained by the automatic control according to the invention islower than the maximum load that could be observed in the positioncontrol according to the prior art.

[0151] Besides that, using the invention, it is possible to have a cycletime varying from one blank to another. However, in any case, the cycletime needed with load control remains shorter than the cycle timeobtained previously with conventional position control.

[0152] Implementation of the invention also results in a variation inthe tool advance rate, as a function of the actual strength of the blankbeing rolled.

[0153] Induced consequences result from all of that. The first is theabsence in fluctuation between the actual distance between centres andthe measured distance between centres because ΔX=f(F)/K, which can beconsidered as a constant, in so far as the load is constant. It is alsofound that there is less work hardening of the press sintered material,for an equivalent reduction in diameter, because the cycle time isshorter.

[0154] Furthermore, the fluctuations in speed and rate are low enoughthat the roundness can be suitably mastered. In consequence, there isless spread, from one blank to another, in the parts obtained, or inother words, the rolling is more repeatable.

[0155] Other experiments were carried out.

[0156] Considered first of all was the rolling of gears, for example a28-tooth helical gear with an actual module m_(n)=2, an actual pressureangle α_(n)=15°, and a helix angle β=32° (international notation).

[0157] These parts are difficult to produce with position controlrolling. Various tests have been carried out with load control rolling,with the terminal planishing phase using position control.

[0158] The Applicant Company has looked for conditions corresponding toa reduction in diameter on the flank and on the root diameter, in orderto achieve the diameters fixed to the plane of definition, frompreformed blanks of different types and geometries. Variations in thedensification thickness stem from this.

[0159] While looking for the maximum limits, the Applicant Companyobserved, for a reduction in diameter of as much as 0.5 mm at the rootof a tooth, that there was premature breakage of the rolling tools,after a few dozen parts had been rolled (something which is economicallyunacceptable) whereas, for its part, the programmed rolling load was asmuch as about 3 500 daN. That stems from the existence of excessivelyhigh mechanical stresses on the teeth of the tools, hence causing theteeth to break at their roots.

[0160] From that, the Applicant Company observed that there was acritical load, relating in particular to the integrity of the tools.This critical load can be achieved by altering the necessary usefulload, which in this particular instance was set at a maximum value of 2300 daN. The blanks were also modified to reduce the densificationthickness, restricted to 0.3 mm at the roots of the teeth, in thisexample. Having made these modifications, it was possible to obtainsatisfactory rolling conditions.

[0161] If, with these new rolling parameters, an oversized blank isintroduced:

[0162] the time (that is to say the number of revolutions of the part orblank) will be increased;

[0163] a safety feature may stop the machine if the cycle becomes toolong;

[0164] the tools do not break.

[0165] Other experiments were carried out for the rolling of discs, forexample of diameters of 35 mm and widths of 10 mm.

[0166] Conventional rolling was carried out with position control, so asto obtain a final dimension of 34.50 mm. The result was that parts brokeand ruptured into multiple fragments. The analysed cause was a spread ofhardness between blanks. This resulted in a final load which varied fromone part to another, occasionally adopting excessively high values whichcaused the aforementioned breakage. A possible remedy (known in positioncontrol) in such instances is to reduce the rate of advance. However,this results in an excessively long rolling time (or number ofrevolutions of the parts) which results in excessive surface workhardening of the part or blank, and in it breaking up.

[0167] On the other hand, by using load control according to the presentinvention, satisfactory parts are obtained. The maximum load is limitedbut constant. The consequence is a reduction in diameter which is morerapid at the start of rolling, on material which has not yet been workhardened. It is then possible to work over a number of revolutions whichon average is lower, and therefore with less work hardening overall.

[0168] The invention is not restricted to the embodiments described.Thus, alternative forms of the machines described in FIGS. 1 to 4 can beused, and in particular:

[0169] two motors for driving the tools O1 and O2, respectively, with orwithout a mechanical connection between their reduction gears,

[0170] two motors for driving the screw/nut systems BSD1 and BSD2 inFIG. 3, two rams for moving the two carriages F1 and F2 with respect tothe frame B in FIGS. 1 and 2,

[0171] adaptations for machines with 3 or more tools.

[0172] More generally, the load control described can be applied to therolling of parts according to variable facilities and implementations,on the basis of the techniques currently applied, or of others yet toarise in this field, such as linear motors, for example. Of course, thatcan be accompanied by various techniques for obtaining load control. Themeasurement quantities are not necessarily loads: it has been seen thatit is possible, in particular, to use pressures, this being merely onenon-limiting example. The action quantities are not necessarily loadseither, so long as it is known how to connect them to loads or forceswith the required precision.

[0173] The invention also covers the essential element which constitutesa programme for operating a numerical control machine for carrying outthe method, in all its alternative forms described.

[0174] Annex 1 TABLE 1 Phase X F rpm; t dX/dt dF/dt ω a XMINa < X < Xa₁F Fa₁ — Ca — ωa b_(i) X > XMINb₁ Fa₁ → Fb₁ Tb — (Fb₁ − Fa₁)/Tb₁ ωb₁ (Fb₁− Fb_(i−1))/Tb_(i) b_(n) X = Xb F = Fb — — — ωb X > XMINb c X = Xc — Tc— — ωc

1. Method of cold forming by rolling of a blank made of press sinteredmaterial, in which at least one tool of predetermined peripheralgeometry is brought close to the blank, so that the tool can then berolled over the blank, urging the one towards the other, characterizedin that it comprises, after a phase (a) of approaching the blank, apenetration phase (b), with: (b_(n)) at least one phase of rolling underroughly constant load, as far as a chosen position, this load, thechosen position, and the corresponding number of passes being determinedso as to control the surface densification and the dimensions of therolled part.
 2. Method according to claim 1, characterized in that thepenetration phase (b) takes place at least partially under load control.3. Method according to one of claims 1 and 2, characterized in that thephase (b_(n)) of rolling under roughly constant load is preceded by:(b₁) at least one phase in which the rolling load increases, bounded bya maximum value of this rolling load.
 4. Method according to claim 3,characterized in that the increase in load in phase (b₁) is also boundedin terms of the progression of the load over time.
 5. Method accordingto one of claims 3 and 4, characterized in that the increase in load inphase (b₁) is carried out according to a critical law which tends tobring the progression close to an experimentally determined permissiblelimit value that takes account of the geometric and mechanicalproperties of the blank and of the finished part.
 6. Method according toone of claims 1 to 5, characterized in that phase (b) includes keepingthe load applied to the blank below a limit value defined with respectto a threshold at which the press sintered blank and/or the toolsdeteriorate.
 7. Method according to claim 6, characterized in that phase(b) includes keeping the load applied to the blank at a value closeenough to the said limit to avoid excessive work hardening while at thesame time minimizing the rolling time.
 8. Method according to one ofclaims 1 to 7, characterized in that the approach phase (a) and thepenetration phase (b) are repeated once the direction of rotation of thetool or tools has been reversed.
 9. Method according to one of thepreceding claims, characterized in that it further comprises: (c) afinishing phase in which the relative positions of the blank and of thetool are kept roughly constant for a chosen length of time.
 10. Methodaccording to one of the preceding claims, characterized in that theperiphery of the tool or tools is roughly circular or generallycylindrical.
 11. Method according to one of the preceding claims,characterized in that the blank is preformed, particularly with teeth.12. Method according to claim 11, characterized in that the tool ortools are equipped with teeth.
 13. Method according to one of claims 1to 10, characterized in that the blank is preformed as a ring,particularly a ring of a bearing.
 14. Method according to claim 13,characterized in that the tool or tools have a uniform externalperiphery.
 15. Program for operating a numerically controlled machine,for carrying out the method according to one of the preceding claims.